1. Field of the Invention
The present invention relates to a paste composition for aluminum brazing excellent in storage stability, applicability, and brazing property.
2. Description of Related Art
Members made of aluminum or aluminum alloys are employed, for example, for automotive aluminum heat exchangers represented by an evaporator, a condenser, and the like to be disposed in vehicles. At the time of brazing these members, compositions containing binders to be mixed to brazing fluxes or mixtures of fluxes and brazing filler metals for even adhesion to joining parts are applied to brazing parts and thereafter, assembling process and heating is carried out to execute a brazing work.
There are following conventional techniques (1) to (6) for such compositions for aluminum brazing.
(1) Japanese Patent No. 2681380
Disclosed is a brazing composition for aluminum or aluminum alloys containing a metal powder for brazing, an acrylic resin with a molecular weight of 1,000 to 100,000 (binder), and either water or an organic solvent and having a solution viscosity of 10 to 5,000 cps (claim 1 to claim 3 in Claims).
However, in the invention disclosed in the above-mentioned Japanese Patent No. 2681380, it is allowed to use water as a solvent. If water is used, aluminum alloys and water cause reaction with the lapse of time to generate hydrogen and thus there is a problem of storage stability and safety. Further, if the solution viscosity is 10 to 5,000 cps (mPa·s), the metal powder for brazing is precipitated with the lapse of time and also it is a problem in terms of storage stability. Furthermore, at the time of application to a mother material, since the viscosity is low, there is a problem that dripping occurs and thus stable brazing cannot be carried out. Further, in order to carry out brazing, it is required to additionally supply fluxes and the operation is complicated (see lines 22 to 24 of fourth paragraph).
Furthermore, in the invention of the above-mentioned Japanese Patent No. 2681380, since the acrylic resin is used as a binder, there is a problem that the brazing property is deteriorated (see Comparative Example 4 described later).
(2) Japanese Patent No. 2873695
A manufacturing method of a heat exchanger using brazing compositions for aluminum or aluminum alloys similar to those described in the above-mentioned Japanese Patent No. 2681380 is disclosed (claim 1 to claim 3 in Claims).
However, Japanese Patent No. 2873695 also has similar problems as those of the above-mentioned Japanese Patent No. 2681380.
(3) Japanese Patent No. 3247150
A composition for brazing containing a metal powder for brazing, a binder resin, and a C1-C8 aliphatic alcohol is disclosed (claims 1 and 4).
Examples of the above-mentioned binder resin include acrylic or methacrylic resins with a molecular weight of 1,000 to 100,000 and butyl acrylate is exemplified in Examples 1 to 9 (paragraph 9 and paragraphs 23 to 32).
However, according to the above-mentioned Japanese Patent No. 3247150, a test result that if a C1-C8 aliphatic alcohol (such as methanol, ethanol, propanol, and the like; claim 3) is used as a solvent for a composition for brazing, the brazing property becomes good as compared with that in the case of using another solvent is described (see Table 1); however actually there is a problem that the brazing property is deteriorated (see Comparative Example 4 described later).
(4) Japanese Patent No. 3705939
A paste composition for aluminum brazing containing 30 to 70% by weight of a metal powder for brazing, 2 to 30% by weight of a flux, 1 to 15% by weight of butyl rubber and/or petroleum resin (C5 type, C9 type petroleum resin: paragraph 11) (binder), and an organic solvent as the balance (0.1 to 67% by weight: paragraph 12) is disclosed (claim 1). The organic solvent as used herein may be those in which butyl rubber is soluble and toluene, hexane, cyclohexane and the like are exemplified (paragraph 12).
However, in the invention described in Japanese Patent No. 3705939, since butyl rubber is used as a binder, there are problems of having malodor and dripping (see Comparative Example 5 described later).
(5) Japanese Unexamined Patent Publication No. 2001-225185
A paste composition for aluminum brazing containing 10 to 70% by weight of a metal powder for brazing, 2 to 50% by weight of a flux, 1 to 15% by weight of a binder, and an organic solvent as the balance (0.1 to 87% by weight: paragraph 17) is disclosed (claim 1). Examples of the binder as used herein include a mixture of an acrylic resin and butyl rubber, or an acrylic resin and petroleum resin (C5 type, C9 type petroleum resin), and the like (claim 1, paragraph 16). Further, the organic solvent as used herein may be those in which a binder is soluble and toluene, hexane, cyclohexane and the like are exemplified (paragraph 17). In addition, there is a description that the above-mentioned composition for brazing may contain various kinds of additives such as a thickener, a plasticizer, and the like (paragraph 23).
However, in the invention described in Japanese Unexamined Patent Publication No. 2001-225185, it is required to use a butyl rubber or a petroleum resin together with an acrylic resin since the brazing property is deteriorated if the acrylic resin is used alone as a binder (see Comparative Example 2 in Table 2), and due to that, there is the following problem. That is, compatibility of a resin becomes worse, which causes deterioration of applicability due to gelation of the resin.
(6) Japanese Unexamined Patent Publication No. 7-155987
A paste composition for brazing aluminum members containing 100 parts by weight of a metal powder for brazing, 0.5 to 15 parts by weight of a flux, 0.5 to 50 parts by weight of a binder composed of a polyalkylene oxide type resin with an average molecular weight of 100,000 to 5,000,000, and 30 to 150 parts by weight of an organic solvent is disclosed (claim 1, paragraph 31). Further, it is described that the paste for brazing may contain a wax, a hardened oil, a hardened caster oil, a fatty acid amide, a polyamide and the like as a viscosity adjustment agent, and the viscosity of the paste is generally 5,000 to 500,000 cps/23° C. (paragraph 29).
However, the invention described in Japanese Unexamined Patent Publication No. 7-155987 uses a polyalkylene oxide type resin as a binder, thereby thermal decomposition property is essentially inferior. Accordingly, there is a problem that a black residue occurs when the atmosphere (oxygen concentration) at the time of brazing is relatively high.